On Wed, 12 Oct 2005 10:47:56 +1300, "Jeff"
wrote:

Hi,
Im confused regarding the choice of iron powder toroid for a 7Mhz high pass
filter. I need to use a small size (T68) toroid. For the freq range, some
reference texts say T68-2 is the one, others say T68-6" grade. I see
differing "best Q range" specs for the 6
grade.....2-30Mhz......10-50Mhz...etc.
Have similar need for 20Mhz LPF, but assume "6" is really the choice there.
Any suggestions as to the relative differences and reasons to choose between
grades 2 and 6 for the HPF in T68 size?
Thanks,

http://www.micrometals.com/

Thanks for all the suggestions. Ive got some Amidon T68-6 toroids to wind
up, and will test the completed filter on a spectrum analyser next week.
JEFF

"Jeff" wrote in message
...
Hi,
Im confused regarding the choice of iron powder toroid for a 7Mhz high
pass
filter. I need to use a small size (T68) toroid. For the freq range, some
reference texts say T68-2 is the one, others say T68-6" grade. I see
differing "best Q range" specs for the 6
grade.....2-30Mhz......10-50Mhz...etc.
Have similar need for 20Mhz LPF, but assume "6" is really the choice
there.
Any suggestions as to the relative differences and reasons to choose
between
grades 2 and 6 for the HPF in T68 size?
Thanks,
JEFF

Hi Guys,
Results of the build and test were that the 7Mhz HPF worked very well with
the "6" grade cores, but the 20Mhz LPF was crap (it used #6 as well). It
started to roll off at 20Mhz as expected, dropped smoothly to -10dB by the
time it got to 40Mhz, then stayed at 10dB at all frequencies above 40Mhz.
I suppose the core should be a grade #10 or #12 instead of #6? or perhaps
just a normal air core?
JEFF


"Jeff" wrote in message
...
Thanks for all the suggestions. Ive got some Amidon T68-6 toroids to wind
up, and will test the completed filter on a spectrum analyser next week.
JEFF

"Jeff" wrote in message
...
Hi,
Im confused regarding the choice of iron powder toroid for a 7Mhz high
pass
filter. I need to use a small size (T68) toroid. For the freq range,
some
reference texts say T68-2 is the one, others say T68-6" grade. I see
differing "best Q range" specs for the 6
grade.....2-30Mhz......10-50Mhz...etc.
Have similar need for 20Mhz LPF, but assume "6" is really the choice
there.
Any suggestions as to the relative differences and reasons to choose
between
grades 2 and 6 for the HPF in T68 size?
Thanks,
JEFF

"Jeff" wrote in message
...
Hi Guys,
Results of the build and test were that the 7Mhz HPF worked very well with
the "6" grade cores, but the 20Mhz LPF was crap (it used #6 as well). It
started to roll off at 20Mhz as expected, dropped smoothly to -10dB by the
time it got to 40Mhz, then stayed at 10dB at all frequencies above 40Mhz.
I suppose the core should be a grade #10 or #12 instead of #6? or perhaps
just a normal air core?
JEFF

Then you haven't built the filter that you designed. The choice of core
material will not affect the blow-by. If you measured the inductors at a low
frequency, I would be pretty certain that you have way too much inductance
at 20-40 MHz.

W4ZCB

From: "Jeff" on Tues, Oct 11 2005 2:47 pm


Im confused regarding the choice of iron powder toroid for a 7Mhz high pass
filter. I need to use a small size (T68) toroid. For the freq range, some
reference texts say T68-2 is the one, others say T68-6" grade. I see
differing "best Q range" specs for the 6
grade.....2-30Mhz......10-50Mhz...etc.
Have similar need for 20Mhz LPF, but assume "6" is really the choice there.
Any suggestions as to the relative differences and reasons to choose between
grades 2 and 6 for the HPF in T68 size?

Been there, done that. In the practical case, what you can expect
is a very slight change in insertion loss in the passband, perhaps
a less sharp transition of attenuation at cutoff, depending on the
type of highpass. It can be modeled in any SPICE analysis program
with accuracy if you make a special model that adds a series R
element computed from reactance divided by Q.

For a highpass filter, the usual filter type has the inductor in
shunt. In that configuration the inductor has a reactance
directly proportional to frequency and will have little effect on
the insertion loss in the passband. The equivalent series R due to
Q won't matter much at 1/3 to 1/4 the cutoff frequency. It might
matter on attenuation in the stopband region but the analysis on
that is more difficult and a practical build-and-measure is the
quicker way to go.

For a lowpass filter, the usual configuration has inductors in
series and there the Q of the inductors will affect insertion loss
more. Those would use the lower frequency range for powder mix.

In a highpass filter it is important that the capacitors have a
minimum series inductance so that the 5x to 10x cutoff frequency
isn't disturbed. Since capacitors are usually in series, their
Q will effect the insertion loss. Fortunately, most capacitors
will have a good Q up around 500+ and won't be a factor.

Use what you have and measure the results (you've got a spectrum
analyzer handy so that's taken care of).

I made a fairly good, practical Synthesis-Analysis program for
L-C filters that includes automatic modeling of (separately) all
capacitor Qs and inductor Qs. I can attach that to private mail
if you want it. Freeware. Proven by practical test comparison.